Liquid esters of neoalkylpolyols and mixtures of gem and straight chain or alkanoic neo acids

ABSTRACT

NEOLKYLPOLYOL ESTERS OF MIXTURES OF GEM ACIDS AND STRAIGHT OR BRANCHED CHAIN NEO FATTY ACIDS ARE PREPARED. THESE ESTERS POSSESS IMPROVED OXIDATION RESISTANCE AND GOOD LOW TEMPERATURE PROPERTIES AND ARE SUITABLE FOR USE AS BASE FLUIDS OR BLENDING STOCK FOR HIGH TEMPERATURE LUBRICANTS.

United States Patent Oflice 3,564,044 LIQUID ESTERS OF NEOALKYLPOLYOLSAND MIXTURES OF GEM AND STRAIGHT CHAIN OR ALKANOIC NEO ACIDS Tai S.Chao, Homewood, and William D. Hoffman, Park Forest, 11]., and ManleyKjonaas, Hammond, lnd., assignors to Sinclair Research, Inc., New York,N.Y., a corporation of Delaware No Drawing. Continuation-impart ofapplication Ser. No. 557,898, June 16, 1966. This application Oct. 5,1967, Ser. No. 673,020

Int. Cl. C07c 69/32 US. Cl. 260-488 10 Claims ABSTRACT OF THE DISCLOSURENeoalkylpolyol esters of mixtures of gem acids and straight or branchedchain neo fatty acids are prepared. These esters possess improvedoxidation resistance and good low temperature properties and aresuitable for use as base fluids or blending stock for high temperaturelubricants.

This application is a continuation-in-part of application Ser. No.557,898 filed June 16, 1966, now U. S. Pat. No. 3,441,600.

This invention is concerned with neoalkylpolyol esters of mixtures ofgem and straight or branched chain alkanoic neo acids. These esters aresuitable, for instance, as base fluids or blending stock in hightemperature lubricants.

The physical state of lubricants, i.e., whether they are fluids orsolids under standard conditions, along with their oxidative and thermalstability is extremely important for high temperature lubricants such asthose used in jet aircraft, since they must be able to flow and pour atsubzero temperatures and at the same time function at high operatingtemperatures.

Lubricants composed in whole or in part of synthetic components havebeen developed in an effort to obtain superior lubricating compositions.In general, these lubricating compositions are characterized by higherviscosity indices, lower pour point and greater heat stability thanmineral oils of corresponding viscosity. Such properties are of specialvalue in lubricating engines which are subjected to high temperatures,such as combustion turbine engines.

Mineral oil lubricants, for example, event those containing added VIimprovers, pour point depressors, or other additives are undesirable foruse in such engines because of the relatively high volatility, low flashpoint, and poor thermal and oxidative stability of the mineral oil whichalso has a tendency to leave deposits which accumulate and interferewith the operation of the engine.

Certain neoalkylpolyol esters of straight chain fatty acids, e.g.,pentaerythritol tetracaproate, which are known as Type II lubricants,have been employed in jet engines but do not possess the thermal andoxidative stability required of the more advanced engine models. On theother hand, esters derived from neoalkylpolyols and branchedchain neofatty acids, e.g., pentaerythritol tetrapivalate, although having goodoxidative stability, are solids at room temperatures and thus have poorhandling, starting and low temperature properties. Polyphenyl ethers,e.g.,

3,564,044 Patented Feb. 16, 1971 m,m,-bisphenoxyphenoxybenzene, havepour points above 35-40" F. and do not flow and pour at sub-zerotemperatures, as required to achieve their functions, especially duringthe critical start up time for jet engines.

To improve thermal and oxidative resistance of jet lubricants, compoundscomprising aromatic, pseudo aromatic and other inherently stable groupshave been suggested as base fluids. Such materials are polyphenylethers, pyrazines, trizines and phosphonitrile chloride derivatives. Allof the above bear high costs which limit their use, and they areinferior in low temperature properties. This precludes their use in jetplanes and in industrial gas turbine units, which are often exposed tosub-zero temperatures.

Various liquid esters such as di-Z-ethylhexyl sebacate, etc., have beenused commercially in or as lubricants. In particular, esters of alkylneo fatty acids and neoalkylpolyols of 3-5 hydroxyl groups such astrimethylolpropane and pentaerythritol have generally been good basefluids for high temperature synthetic lubricants. The presence of alkylgroups and the absence of I-I-atoms on the a-carbon atom impart manyunique properties to alkyl neo fatty acid esters such as increasedthermal, oxidative and hydrolytic stability. This increased stabilityrendders them highly desirable for use as base fluid for jet enginelubricants, functional fluids, greases and as plasticizers.

However, the presence of the alkyl groups on the a-carbon atom alsoimparts some severe disadvantages. The first disadvantage is due to thesteric hindrance exerted by the alkyl groups on esterification, with theconsequent need for more drastic conditions which increase processingcosts and result in inferior products. The second and much more damagingeffect is that, despite having greater thermal stability than straightchain acid esters, they also have higher melting points.

Further, the successful use as lubricants of esters of alkyl neo fattyacids and neoalkyl polyols has been extremely limited, since most ofthem have been known to exist as solids at room temperature, this beingespecially true for the plural esters such as those wherein 3 or more ofthe hydroxyl groups of the alcohol are esterified.

It has now been found, in accordance with the present invention, thatbase fluids or blending stocks for high temperature lubricants, havingimproved oxidation resistance and good low temperature properties can beprepared by reacting neoalkylpolyols such as pentaerythritol, withmixtures of straight chain or alkanoic neo acids of 4 to 10 carbon atomsand preferably 5 to 8 car-bon atoms and certain well chosen gem alkanoicacids having the general structural formula:

wherein R R and R are C -C alkyl groups, n is 1 to 3 and the totalnumber of carbon atoms is not more than 9. \Suitable gem acids include,for example, 3,3-dimethy1- pentanoic, 4,4-dimethylpentanoic,3,3-dimethylbutanoic, 4,4 dimethylhexanoic, 5,5 dimethylhexanoic,3,3,4-tr1- methylhex-anoic and 4,4,S-trimethylhexanoic acids.

The straight chain or alkanoic neo acids are usually employed in minoramounts in mixture with the gem acid, but may be employed in amounts offrom 5 to 50% by weight and preferably at least 10% The gem acids can beprepared by known processes for preparing aliphatic acids. For example,3,3-dimethylpentanoic acid can be prepared from 3,3-dimethylpentanol byoxidation With KMnO While 4,4-dimethylpentanoic acid can be prepared bytreatment of 3,3-dimethyll-chloro-butane with NaCN, followed by acidhyrolysis.

It may be seen that these gem acids differ from neo acids, such asneopentanoic or pivalic, neoheptanoic, neodecanoic, acids, etc., in thatthe neo acids have no -CH groups between the branching and the COOHgroup.

The gem acids of this invention also differ from other branched-chainacids in that they contain at least one gem group:

l Ra

and contain no tertiary H on the a-C atom. By tertiary H on the oc-Catom, it is meant the H shown in the formula:

The R is an alkyl group.

can be carried out in the same manner as the esterification of straightchain acids alone. No drastic conditions, such as the use of relativelyhigh percentages of H 50 or the conversion to acid chloride are needed.In fact, for the preparation of esters for use in synthetic lubricants,the use of H 50 p-toluene sulfonic acid and other S-containing catalystsshould be avoided. Even a trace of sulfur will result in deteriorationof the quality of the finished product.

As may be seen, improved low temperature properties were obtained byincorporating a minor amount of a straight chain fatty acid, such asn-valeric acid, during the esterification of pentaerythritol with3,3-dimethylpentanoic acid. By this means esters of lower pour points,lower viscosity at low temperatures and excellent low temperaturestorage stability were obtained. Table I shows the properties of foursuch esters. It can be seen that a pour point of F. was obtained withthe incorporation of 18.5 mole percent (15.1 wt. percent of n-valericacid in the acid mixture and one of F. was obtained with 28.0 molepercent (24.2 wt. percent) of n-valeric acid. The viscosity at 0 F. wasreduced to 17,229 and 8,997 cs. respectively. Further reduction of KV at0 F. to 3,851 cs. was observed when 48 mole percent (42 wt. percent) ofn-valeric acid was used. The mixed esters so prepared remained as clearliquids after seeding (with the tetraester) and storing in refrigerator(20 F.) and in low temperature baths (at 0, -30, --40, and 50 F.) forover hours.

TABLE I.PROPERTIES OF PENTAERYTHRITOL ESTERS OF MIXTURES OF3,3-DIMETHYLPENTANOIC ND VALE RIC ACIDS Pentaerythritol3,3-Me2-pentanoic acid n-Valeric acid Mole percent n-valeric acid inacid mixture .I- Mole percent of n-valcric group in ester, by N MR.

Kinematic viscosity, 0. s.:

Pour point, Cloud point,

Flash point, F. Fire point, F... Melting point, F

Saponification Number 422.7 426.1 .I 4221 Low temperature storage testappearance after 60 hrs.:

20 F Solid Clear liquid--- Clc 1\ Too viscous to measure. b supercooled.

After seeding and cooling in refrigerator. Without seeding a samplestayed liquid for 2 years at room temperature.

The neopentylpolyols suitable for the purpose of this invention areorganic compounds often having up to about 12 carbon atoms with 2-6hydroxy groups or more and preferably not more than 7 carbon atoms with24 hydroxy groups and containing at least one neopentyl group and atleast two hydroxy groups attached to the carbon atoms next to thequaternary C atom. This structure is as follows:

I C HOCH2(IJCH2OII I Examples of such compounds include, for example,neopentyl glycol, 1,1,l-trimethylolethane, 1,1,l-trimethylolpropane(TMP), 1,1,1-trimethylolbutane, pentaerythritol (PE), dipentaerythritol,etc.

Esterification of the mixed gem alkanoic acids and straight chain oralkanoic neo acids with neopentylpolyols Although liquid esters can beprepared from neoalkylpolyols with mixtures of neo and straight chainacids, our studies indicated that a much higher percentage of thestraight chain acid had to be used, with a corresponding 0 sacrifice inoxidation resistance. Thus, as may be seen in Table II, pentaerythritolesters of mixtures of neoheptanoic and n-valeric acids containing lessthan 50 mol percent n-valeric groups (as determined by NMR) areinvariably solids. Even those containing more than 50 mole percentn-valeric groups showed poor low temperature stability. This is becausethe ester product is usually a mixture containing all possible singleesters such as tetra-neo-C tetra-n-C tri-neo-C -mono-n-C di-neo-C di-n-Cetc. Tetra-neo-C having a M.P. of 189l92 F. will crystallize out of thismixture upon standing, especially at low temperatures. Esters ofmixtures of gem-C and n-C acids, on the other hand, did not show thistendency of crystallization. It is believed that this is due to the lowmelting point (-82 F.) of pure PE tetra-gem-O, and its strong tendencyto remain as a super-cooled liquid.

TABLE II.PROPERTIES OF MIXED PENTAERYTHRITOL ESTERS OF NEOHEPTANOIO ANDN-VALERIC ACIDS Sample No. A B O D E Mole percent neo-0 Calculated 70Found (NMR) 100 73.9 60 8 Kinematic viscosity, cs.:

0F 8.171 6.469 5.712 5.245 4.744. 100 "F Solid Solid 42. 22--.- 35. 8929 0 F 40 F Pour point, F Cloud point, Flash point, F 440 455 Firepoint, F 500 520 6-0. 520. Melting point, F 189-192 135-144" 86-96 Lowtemp. stability, appearance after 60 hrs.:

F Solid So1id Solid Liquid and Clear crystals. liquid do Do. Do. Do. Do.Do.

n Too viscous to measure.

The excellent low temperature properties of mixed esters of gem andstraight chain acids of this invention were also found with the estersof mixtures of gem and alkanoic neo acids. A pentaerythritol ester,prepared from 0.25 mole of pentaerythritol (PE), 0.75 mole of gem-C acidand 1.0 mole of neo-C group was found by NMR to contain 74.2 molepercent gem-C and 25.8 mole percent neo-C groups. It had a KV/210 F. of11.91 cs., KV/ 100 F. of 181.0, KV/0 F. of 88,620 cs., pour point of -20F., flash point of 485 F. In low temperature storage tests it stayed aliquid at 20 and 0 F. for more than 60 hours. These results aresummarized in Table III.

TABLE III Properties of mixed pentaerythritol ester of3,3-dimethylpentanoic acid (gem-7 acid) and neoheptanoic acid (neo-7acid) Identity PE-gem-C -neo-C Kinematic viscosity, cs.:

210 F 11.91. 100 F 181.0. 0 F 8 8620. -40 F Pour point, F. 20. Cloudpoint, F. None. Flash point, F. 485. Fire point, F. 560. Melting point,F Mole percent gem-C 74.2. Low temp. stability, appearance after 60hrs.:

20 F. Liquid. 0 F. Liquid. F. Glassy solid. F. e Glassy solid. F. Glassysolid.

The oxidation resistance of the esters of this invention was determinedby two standard tests which have been correlated; well with gas turbineengine performance. The Oxygen Absorption Test measures the rate of Oabsorption in the presence of a standard inhibitor (1% by wt. ofN-phenyl-l-naphthylamine), as well as the increase in viscosity and acidnumber after the absorption of 2500 ml. of oxygen and is a test todetermine the oxidation resistance of lubricants. In principle itresembles the Dornte Oxygen Absorption Test (see WADC Tec. Report 59-191, part III, pp. 30-32) and measures the rate of oxygen consumption bythe lubricant at a given temperature.

Briefly, a Weighed sample of the lubricant is placed in a Pyrexoxidation cell which is heated electrically in an aluminum block. Oxygengas is bubbled through the lubricant at a specified rate. The exit gasesgo through a series of absorbants and a tube furnace so that all C0, C0H 0 and organic materials are removed, leaving only the unconsumedoxygen which is circulated again through the hot lubricant. As thelubricant is oxidized and oxygen is usedup, the pressure in the closedsystem drops. This pressure drop which is directly proportional to thevolume of O consumed is constantly monitored and recorded. A curve isobtained which indicates the volume of oxygen consumed vs. time. Alubricant of good oxidation resistance usually shows a very low rate ofoxygen consumption at the beginning. As oxidation proceeds theinhibitors will gradually be used up and the pro-oxidation species (suchas peroxy radicals) will accumulate. At one point the rate of oxidationwill increase drastically and the curve makes a sharp bend. The timerequired for this to happen is generally called induction time (T When aspecified volume (V of 0 has been consumed, the unit is shut downautomatically. The time for this to happen is called total time (T Theused oil is then analyzed for acid number and increased viscosity (A(KV/or A(KV/2l0) for solid esters) and, sometimes, pentane insolubles.Therefore, this test also furnishes information on the extent ofoxidative degradation (as indicated by increases in acid number andviscosity) upon the consumption of a definite amount of oxygen by thelubricant. This last-named property is often called oxygen tolerance. Alubricant of good oxygen tolerance and low rate of oxidation (high T andT,) generally will show less oxidative degradation in a bearing rig andin service. The amount of stain or deposit on the wall of the oxidationcell can often be used to predict the cleanliness of the lubricant inrig tests, although this relationship is not as reliable as therelationship mentioned above.

Table IV shows the results of the Oxygen Absorption Tests for esters forthis invention (samples 1 and 2) and compares them with results obtainedfrom tests of other neoalkylpolyl esters. As may be seen thepentaerythritol ester of the mixed C gem and neo acids (sample 2) gavesignificantly higher induction time (T and total times (T than the otheresters while the other ester of this invention, the pentaerythritolester of a mixture of 81.5% by weight C gem acid and 18.5% normal C acid(sample 1) was inferior in this respect only to samples 3 and 5.

TABLE IV.-02 ABSORPTION TEST RESULTS OF NEOIENTYLPOLYOL ESTERS SampleTi, T v,, v,, A(KV/210), A(KV/100), Number Ester min. min. m]. mi.percent percent A (A.N.)

1 PE gem-CT-n-C (containing 81.5% gem-C 709 759 815 14.12 2-... PEge1n-C -neo-C1 951 993 1,470 11.9 3 PE tetra-neo CL 723 308 410 5. 564.- PE tetra-nC 156 202 307 7. c3 5 PE tetra-gem-Cr... 733 772 1,3609.39 6.. PE tetra (4, 4-dirnethyll1cxanoate) 271 303 524 10.61 7 PEtetra (5, S-dimcthylhexanoate) 332 368 1, 200 14. 8 TMPtri(4,4-dimethylhexanoate) 300 351 12.49 9 TMP tri(4,4,-trimethylhexanoate) 159 212 449 9.26

Another important test for oxidation resistance is the (a) a gemalkanoic acid having the structure:

Erdco High Temperature Bearing Head Test. This test R2 f r th 1' I hasbeen used by yet engine manu acture s, syn e 1c RI C (CHZ)B COOHlubricant produces and govenment agencies as the final I screening toolof candidate lubricants before the engine test. For a Type 2 /2lubricant, the temperatures used in the test are 50 F. higher than theType 2 test. Table V shows the test results of three high temperaturesynthetic lubricants. Lubricant A is a Type 2 synthetic lubricant whichpassed the Type 2 bearing rig test and which comprises a mixture ofstraight chain carboxylic acid esters of monoand dipentaerythritol. Itgave a \WADD rating of 73 and viscosity rise of 69% in the Type 2 test.When it was tested under 2 /2 conditions, the viscosity increase was sohigh (829%) that the test had to be terminated at 75 hrs. The WADDrating was 190.5. Using the same additives but replacing the base fluidwith PE-neo-C -n-C (containing about 43.2% neo-C groups), lubricant Bwas obtained. The test was completed, but there was a high viscosityrise (250%) and a large amount of filter deposits. Lubricant C, whichhad the same additives as A and B but with PE-gem-C -n-C (containing81.5% gem-C as base fluid, showed significant improvements in P&Wrating, WADD rating, filter deposit, viscosity increase, as well as thefinal acid number. Since the three lubricants and the same additives,the difference is due to the difference in oxidation resistance of thebase fluid. The base fluid for Lubricant C is superior to that ofLubricant B because the former contained a much smaller proportion ofstraight-chain groups which are more susceptible to attack by oxygen.Any reduction of the percent of straight-chain groups in the base fluidfor Lubricant B will result in solids unsuitable for use in gas turbineengines.

TABLE V.-TYPE 2% ERDCO BEARING HEAD TEST RESULTS Conditions:

Duration. hrs.100 Test bearing temp. F.550 =1: 5 Oil in temp., 13- 150:1: 10 Sump temp, F.490 :i: 5

1. A liquid ester formed from a neoalkylpolyol having up to about 12carbon atoms with 2 to 6 hydroxy groups and a mixture of acidsconsisting essentially of:

wherein R R and R are C to C alkyl groups, n is 1 to 3 and the totalnumber of carbon atoms is not more than 9; and

(b) an alkanoic acid selected from the group consisting of straightchain and alkanoic neo acids of 4 to 10 carbon atoms, and in amount of 5to 50 percent of said mixture of acids.

2. The liquid ester of claim 1 in which the alkanoic acid selected fromthe group consisting of straight chain and alkanoic neo acids is of 5 to8 carbon atoms and is in amount of 10 to 50 weight percent of saidmixture of acids.

3. The liquid ester of claim 1 in which the neoalkylpolyol is aneopentylpolyol.

4. The liquid ester of claim 3 in which the neopentylpolyol ispentaerythritol.

5. The liquid ester of claim 4 in which the gem alkanoic acid is3,3-dimethylpentanoic acid and the alkanoic acid (b) is n-valeric acid.

6. The liquid ester of claim 4 in which the gem alkanoic acid is3,3-dimethylpentanoic acid and the alkanoic acid (b) is neoheptanoicacid.

7. The liquid ester of claim 2 wherein the neoalkylpolyol ispentaerythritol and the gem alkanoic acid is selected from the groupconsisting of 3,3-dimethylpentanoic, 4,4-dimethylpentanoic,3,3-dimethylbutanoic, 4,4- dimethylhexanoic, 5,5-dimethylhexanoic,3,3,4-trimethylpentanoic and 4,4,5-trimethylhexanoic acids.

8. The liquid ester of claim 7 wherein the straight chain and alkanoicneo acids are selected from the group consisting of n-valeric acid andneoheptanoic acids.

9. The liquid ester of claim 8 wherein the gem alkanoic acid is3,3-dimethylpentanoic acid.

10. The liquid ester of claim 1 wherein the neoalkylpolyol has no morethan 7 carbon atoms with 2 to 4 hydroxy groups and contains at least oneneopentyl group and at least two hydroxy groups attached to the carbonatoms next to the quaternary carbon atom.

References Cited UNITED STATES PATENTS 2,798,083 7/1957 Bell et al.260--410.6 2,991,297 7/1961 Cooley et al. 1111 2,991,297 7/1961 Cooleyet al. 26041O.6 2,820,014 1/1958 Hartley et. al. 260-4106 LORRAINE A.WEINBERGER, Primary Examiner R. S. WEISSBERG, Assistant Examiner US. Cl.X.R. 260410.6

